The general concept of utilizing electrically erasable phase change materials (i.e., materials which can be electrically switched between generally amorphous and generally crystalline states) for electronic memory applications is well known in the art and is disclosed, for example, in U.S. Pat. No. 3,271,591--Ovshinsky, issued Sep. 6, 1966 and in U.S. Pat. No. 3,530,441--Ovshinsky, issued Sep. 22, 1970, both assigned to the same assignee as the present invention.
As disclosed in the aforementioned Ovshinsky patents, such phase change materials can be electrically switched between two different structural states of generally amorphous and generally crystalline local order or between different detectable states of local order across the complete spectrum between the completely amorphous and completely crystalline states. That is, the switching of such materials is not required to take place between completely amorphous and completely crystalline states but rather can be in incremental steps of local order changes to provide a "gray scale" represented by a multiplicity of conditions of local order across the spectrum between completely amorphous and completely crystalline states. The materials described can also be switched between only two structural states of generally amorphous and generally crystalline local order to accommodate storage and retrieval of digital information.
The Ovshinsky electrically erasable phase change memories were fully adequate for many applications at the time they were originally introduced and were utilized in a number of applications. However, because further development of that early technology was not possible because of lack of the necessary resources to carry the same forward, subsequent developments in other fields of solid state, electronic memories and in other types of memories in general, such as those utilizing magnetic and optical media, gradually displaced that early electrically erasable phase change technology.
As a result of the aforementioned lack of ongoing development support, there are at the present time several limitations in the electrically erasable memory applications of the Ovshinsky phase change materials which have prevented their widespread use in electrically erasable phase change memories. One of these has been the relatively slow (by present standards) electrical switching speed which such prior art materials have exhibited, particularly in the direction of greater local order or in the direction of increasing crystallization. Another has been the relatively high energy required for initiating the phase change between one state and the other.
For example, the switching times of such prior art phase change materials are typically in the range of a few milliseconds for the set time from the amorphous state to the crystalline state and perhaps a microsecond or so reset time from the crystalline state back to amorphous state. The electrical energy required to switch such prior art materials was typically measured in the range of about a microjoule.
The concept of utilizing the Ovshinsky phase change materials in non-erasable or non-reversible, write-once electrically programmable memories is also well known in the prior art. This type of electrically programmable phase change memory is disclosed, for example, in U.S. Pat. Nos. 4,499,557--Holmberg et al., issued Feb. 12, 1985 and 4,599,705--Holmberg et al., issued Jul. 8, 1986, and assigned to the same assignee as the present invention. The aforementioned Holmberg et al. patents include tetrahedrally chemically bonded materials such as carbon, silicon and germanium and alloys thereof as phase change materials which are utilized in a non-reversible or non-resettable mode. Such materials are disclosed as having, for example, characteristics which require threshold setting voltages of up to 10 volts, currents up to 25 milliamps and setting times of up to 100 microseconds. Thus, the set energy required is up to 250 milliwatts with set times up to 100 microseconds.
Accordingly, because of the lack of ongoing development support, these materials have not found widespread use in reversible or electrically erasable memory applications, where other types of memories offer substantially lower switching times and energies. Instead, other forms of solid state, electronic memories have evolved and have enjoyed some limited use in these applications. These memories typically use several solid state microelectronic circuit elements for each memory bit, as many as three or four transistors per bit, for example, in some memory applications. The primary memory elements in such solid state memories are typically floating gate field effect transistor devices which hold a charge on the field effect transistor gate to store a memory bit. Since this charge can leak off with the passage of time, the storage of information is thus not truly non-volatile as it is in the phase change media where information is stored through changes in the actual structure of the material.
Such solid state, electronic memories which are presently in use are also relatively expensive to manufacture and their cost is typically about twice the cost per bit of storage capacity in relation to magnetic disk storage. On the other hand, solid state, electronic memories have certain advantages over magnetic disk memories in that solid state memories have no moving parts, are easy to transport and store and are more versatile in their adaptability for use with portable computers and other portable electronic devices. In addition, such solid state memories are usually true random access systems as opposed to disk types which require physical movement of the disk head to the proper data track for accessing the desired memory location.
However, in spite of such advantages of solid state electrically erasable memories, their substantially higher costs have prevented them from enjoying a substantial share of the market now dominated by disk type memory systems. Although electrical solid state memories based on phase change materials have shown potential for manufacture at reduced costs, the performance parameters available from such systems as known in the prior art have not been adequate to permit their widespread use as replacements for disk type systems or other solid state memory systems of the type described above.